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Extract: This book started with an apparently simple question that seemed to have a simple answer: How long does it take to form a new habit?

Say you want to go to the gym regularly, eat more fruit, learn a new language, make new friends, practice a musical instrument, or achieve anything that requires regular application of effort over time. How long should it take before it becomes a part of your routine rather than something you have to force yourself to do?

I looked for an answer the same way most people do nowadays: I asked Google. This search suggested the answer was clear-cut. Most top results made reference to a magic figure of 21 days. These websites maintained that “research” (and the scare-quotes are fully justified) had found that if you repeated a behavior every day for 21 days, then you would have established a brand-new habit. There wasn’t much discussion of what type of behavior it was or the circumstances you had to repeat it in, just this figure of 21 days. Exercise, smoking, writing a diary, or turning cartwheels; you name it, 21 days is the answer. In addition, many authors recommend that it’s crucial to maintain a chain of 21 days without breaking it. But where does this number come from? Since I’m a psychologist with research training, I’m used to seeing references that would support a bold statement like this. There were none.

My search turned to the library. There, I discovered a variety of stories going around about the source of the number…